This invention relates in general to fabric printing processes and, more particularly, to processes and apparatus for lithographic and screen printing of fabrics.
Screen printing processes are widely used to print patterns on fabrics used in the manufacture of ties and other wearing apparel. In such processes, colored inks are applied to the fabric through individual framed screens which are made of mesh material and have been prepared in a manner to allow the ink to pass through only those portions of the screen which correspond to the desired pattern. A squeegee or pressurized plenum is typically used to force the inks through the screens and onto the underlying fabric substrate. In order to obtain a multi-colored design, a different color ink is used for each screen and the screens are typically mounted to move in relation to an underlying platen on which the fabric substrate is applied. Because individual portions of the multi-colored design are applied sequentially using multiple screens, great care must be exercised to ensure that each screen is precisely aligned over the fabric substrate during application of the ink to the fabric.
Although it is often desirable to include a large number of colors in a pattern applied to fabrics using screen printing methods, the preparation and use of multiple screens is labor intensive and substantially increases the cost of the printed article. In an effort to reduce screen printing costs while still providing for a range of printed colors, a screen printing process has been developed which utilizes combinations of four base colors, such as black, magenta, yellow and cyan, to achieve the desired color. In general, multiple colors are obtained in this four color process by overlaying one or more base colors over another base color on the substrate, e.g. applying the cyan color over the color yellow will produce a green hue and adding magenta will produce a brown hue.
In order to achieve the desired color pattern in a four color screen printing process, each of the four screens is prepared in a manner so that it will produce a pattern of small ink dots of one of the base colors. One base color of ink is then applied to the fabric substrate through the first screen, with the other base colors being applied successively through the other screens. The sharpness of the resulting image is dictated not only by the alignment or registration of the screens and the type of fabric utilized, but also by the size and spacing of the ink dots.
A primary advantage to the four color printing process is that only four screens are needed to produce the desired pattern. As a result, labor and cost savings are achieved in the preparation of the screens because fewer screens are prepared in comparison to other conventional processes in which up to fifteen screens may be utilized. In addition, the four color process is less labor intensive during the printing process because the inks are applied through only four screens. However, one disadvantage resulting from this process is that exact alignment or registration of each of the four screens is required in order to produce a sharp image. If any one of the screens is slightly misaligned during the ink transfer process, the colors will "bleed" as ink dots are shifted from their intended alignment. This problem becomes even more pronounced in those instances where smaller ink dots are used to obtain sharper images. As a result, the number of dots per inch used in four color screen printing processes and the clarity of the resulting image must often be less than would otherwise be desired.
Another problem associated with screen printing processes, and four color processes in particular, is the color variation that frequently occurs as the ink permeability through the screens changes. The changes in ink permeability can result from a number of factors such as dried ink plugging portions of the screens and variations in the force applied by the squeegee or pressure plenum. While minor color changes can often be tolerated in most production runs, even subtle color variations can significantly detract from the appearance and value of certain articles such as piece goods which are subsequently sewn together to form a finished product.
The inability to precisely align successive fabric substrates during screen printing processes also lessens the production rates that can be achieved in some instances. The alignment difficulties can often be traced to the tendency of certain fabrics to elastically contract, such as when sheets of fabric are cut from large rolls of fabric. Silk is one material which is particularly prone to such movement or warping when it is handled. If the fabric movement occurs prior to printing then the printed image will be distorted. In addition, because the printed patterns are often cut at the same time from multiple sheets of fabric which are stacked together, alignment problems may also result if the fabric moves after printing but prior to cutting of the printed pattern. Even a slight misalignment during the cutting process may adversely affect the appearance and value of the finished product.
Although the use of larger sheets of fabric can reduce the movement of materials such as silk in screen printing processes, the screens used to print the fabric are typically available in a limited number of standard sizes. A significant need thus exists for a method of stabilizing fabrics such as silk so that existing screens can be utilized and still produce the desired high quality printed products.
Four color printing is also commonly used in a type of printing known as lithographic printing. In lithographic printing machines, plates which are typically made of aluminum are wrapped around rotatable cylinders. The plates have been etched in a manner which causes the ink to be retained on only the desired portions of the plate. As a substrate such as paper is fed through the machine, it contacts successive plates which transfer the colored ink dot patterns onto the paper to form the desired multicolored image. Because of the ability to exactly align suitable substrates in lithographic printers, the number of dots per inch which can be used often greatly exceeds that which can be obtained using screen printing processes.
Although internal alignment mechanisms are utilized in lithographic printing machines in order to assist in the alignment of the substrate as it is pulled through the machine, the substrate must possess a certain minimum degree of stiffness and resistance to deformation in order to be properly aligned and to maintain the desired alignment during the printing process. Many types of fabrics, including silk, are generally unsuited for conventional lithographic printing processes because they lack the physical characteristics needed to ensure proper alignment or registration. Thus, while lithographic printing is generally faster and can produce a sharper and more readily reproducible image in comparison to screen printing, lithographic printing remains unsuited for use with many types of fabrics.